A cell or battery gives a direct potential difference (dc). The potential difference stays the same way round, so the current always flows in one direction around the circuit.
Mains electricity is different. It is an alternating potential difference (ac). The potential difference keeps changing direction, rising to a maximum in one direction, falling to zero, then rising to a maximum in the opposite direction. This makes the current in the circuit change direction again and again.
In the United Kingdom the domestic mains supply has a frequency of 50 Hz, which means it completes 50 full cycles every second. The potential difference of the mains is about 230 V.
